This section provides background information related to the present disclosure which is not necessarily prior art.
The principle of friction stir welding is described in WO 93/10935 and involves a rotationally driven tool to join together two abutting workpieces which are typically formed of metallic material. The two metal workpieces may be placed surface against surface (lap joint) or next to one another such that they are butted against each other with their narrow side surfaces (butt joint). The material of the two workpieces is initially plasticized and intermixed due to the frictional heat that results from the movement of the rotating friction stir welding tool in between the two surfaces. After some time the material at the interface of the two metal work pieces cools down and solidifies again such that the two metal work pieces are welded together.
The frictional heat that is generated due to the rotation of the friction stir welding tool in between the two facing work pieces does not only heat up the two abutting surfaces but is also transferred into the friction stir welding tool. The rotating probe and the shoulder element are particularly heated resulting in excessive process temperatures also in the region around the probe and thus in the shoulder element, which significantly reduces the life time of the entire friction stir welding tool.
Apparatuses for friction stir welding are already known from the prior art, such as for example WO 98/51441 or U.S. Pat. No. 6,199,745 B1. These apparatuses usually comprise a probe with an engagement portion for engaging with the two facing work pieces and a shoulder element for bearing against the surfaces of the two working pieces. The probe and the shoulder elements are driven in a rotating manner with a drive shaft.
The tool of WO 98/51441 further comprises a cooling mechanism. A cooling medium is transported into the interior of the friction stir welding tool via supply means. These supply means comprise one elongate conduit that extends through the tool facing directly the probe, which may be consequently cooled down during the welding process. The device described in U.S. Pat. No. 6,199,745 B1 also comprises conduits to supply parts of the interior of the welding tool with a cooling fluid, whereas these conduits do not cross the probe and the shoulder element.